From the state of the art, as exemplified by U.S. Pat. No. 4,311,123, it is known to detect overrun operation of a motor vehicle in that, for a closed throttle flap, the engine rpm exceeds a specific value. This is the case, for example, in downhill driving when the operator removes the foot from the accelerator pedal. Generally, the term overrun operation is, however, broader because it is always then applied when the motor vehicle decelerates only because of a partial release of the accelerator pedal. For this, it is not absolutely necessary that the throttle flap be completely closed. It is only essential that the rpm in overrun operation is greater than that value which would adjust continuously in the case of normal operation for the particular position of the accelerator pedal. It is known, when detecting overrun operation, to reduce the metering of fuel at the start of this overrun operation in accordance with a selectable function and, if required, to reduce the metering of fuel to zero and, at the end or after the end of overrun operation, to again permit the metering of fuel to resume pursuant to a selectable function. The end of the overrun operation is generally detected by a drop below a lower engine rpm threshold (resume rpm).
For motor vehicles equipped with an automatic transmission, a gear stage is selected generally in dependence upon the load state and the speed of the motor vehicle. If the motor vehicle is in the above-mentioned overrun operation, then an upshift operation can be triggered based on the shift characteristic lines. This reduces the braking action of the engine which is wanted in overrun operation. To avoid these disadvantages, upshift operations of the automatic transmission are prevented when there is a jolt-like withdrawal of the accelerator pedal. In this connection, reference may be made to U.S. Pat. No. 4,258,591.
A safety system for motor vehicles is disclosed in German patent publication 4,122,083. In this system, a deceleration of the motor vehicle is indicated by a timely downshift before a pregiven engine rpm limit is exceeded. For this purpose, this safety system warns the driver before the highest engine rpm is reached when driving downhill and when there is an appropriately heavy load so that the driver can perform the required downshift.
The following operating sequence is to be observed for a motor vehicle equipped with an automatic transmission and a fuel cutoff in overrun operation. If one permits a motor vehicle equipped with a spark-ignition engine, gasoline injection and an automatic transmission to simply roll from a higher speed (accelerator pedal position essentially at zero), then an engine rpm adjusts depending upon the gear which is engaged (at high speed, the highest gear as a rule). At high speed, the rpm is usually so high that, when the accelerator pedal is partially released, the injection carries out an overrun cutoff. Generally, this reduces the consumption of fuel to zero. If the roadway does not have a steep drop, the speed and therefore the engine rpm drop slowly until the engine rpm drops below the threshold for resuming injection. On the one hand, the braking action of the engine drops while, on the other hand, the consumption of fuel increases. Since the braking action of the engine is mostly wanted in city traffic (otherwise, the missing deceleration must be increasingly provided by the brakes), the resumption of injection has negative consequences. If the speed drops still further, the automatic transmission shifts into a lower gear. In this way, the engine rpm increases somewhat, however, the downshift thresholds lie so low at low speeds that the rpm always remains below the threshold for the overrun cutoff.